This invention relates to automatic control circuits for contactors in general and more particularly to an improved drive circuit for a main contactor.
Drive circuits for main contactors and the like which are used as individual control means between a central command transmitter and the individual control elements of a control system are known. Typically these may be between the control station in the control room for the automatic control of a functional group or a data processing system. The drive circuits are used to couple the commands of the command transmitter logically to each other and to protection commands and monitoring signals to form positioning commands for the control elements assigned to them.
One known drive circuit for a power contactor not only amplifies the ON and OFF commands of a higher order central command transmitter for transmission to auxiliary or coupling contactors but also contains a monitoring unit which monitors return signals from the main contactor to determine whether or not it has reached the position ordered after an adjustable travel period. That is to say to determine whether the main contactor has opened or closed as commanded. Such a circuit is disclosed in Siemens Zeitschrift 47, 1973, pages 315 to 320. If the contactor does not reach the ordered position or leaves that position without an appropriate command, an error signal is provided to the central command transmitter which then intervenes with appropriate measures.
When using this type of a drive circuit for a contactor having a latching feature, a lowering of the control voltage can lead to an uncontrolled opening of the contactor. Since the return signals of the opened contactor will then no longer agree with the stored positioning command an error is signaled which causes the central command transmitter to intervene. On the basis of the signaled trouble, the central command transmitter can shut off either the entire installation completely or an extensive portion thereof.
However, such a protective disconnection of the entire installation or an extensive part thereof is desirable only if serious trouble actually occurs. Often voltage dips of short duration will occur and then disappear again all by themselves. In such cases a protective disconnection is undesirable. Thus, it can be seen that there is a need for an improved circuit of this nature for controlling a main contactor which will open the contactor in a defined manner should a voltage dip occur and if the voltage dip disappears within a specified time span will reclose the contactor automatically. However in the case of a voltage dip which persists closing action must be blocked.